1. Field of the Invention
The present invention relates to a filter for removing inclusions from molten pure aluminum or molten aluminum alloy (hereinafter, referred to as “molten Al” as a whole).
2. Description of the Related Art
The energy necessary for remelting aluminum is only 1/30 time of that for electrolytic refining of aluminum ore. Therefore, the recycling aluminum scrap (i.e. remelting the aluminum scrap for use as raw material to be melted) can give a considerably large amount of energy savings and thus bring great advantages.
However, the aluminum scrap generally contains various impurities at high concentrations. Thereby, the molten aluminum or molten aluminum alloy obtained by remelting such an aluminum scrap is likely to contain a variety of inclusions including many kinds of non-metallic inclusions. The non-metallic inclusions are, for example, oxides contained originally in the aluminum scrap to be used as the raw material to be melted (hereinafter, referred to simply as “raw material”) or oxides produced by the reaction of the impurities contained in the aluminum scrap with oxygen in the atmosphere during the remelting. Compared with using aluminum ore, the above-mentioned problem of the oxide inclusions is more severe when using the aluminum scrap as raw material. Examples for such oxide inclusions are alumina (Al2O3), spinel (MgAl2O4), and magnesia (MgO).
Consequently, in case of using such an aluminum scrap of aluminum alloy products as a part or all of the raw aluminum material, it is necessary to remove inclusions at least from the molten Al obtained by melting the raw aluminum material. Therefore, in addition to the refining process such as a degassing of the molten Al with using a flux, performed has been a process of removing inclusions from the molten Al such as the killing method, active gas method and molten Al filtering.
According to the killing method, the molten Al is let stand still in an undisturbed state. This still standing floats up inclusions in the molten Al or settles them to the molten Al bottom, because specific weights of the inclusions differ from that of the molten Al. Thus, the separation of the inclusions from the molten Al can be achieved. The separation efficiency, however, is extremely poor. This is because 1) the specific weight difference between each of the inclusions and the molten Al is not satisfactorily large, and 2) the wettability therebetween is considerably large. Consequently, there is the problem that large amounts of the inclusions remain in the molten Al even after a long period of time to complete this method.
According to the active gas method, an inert gas or a halogen gas is introduced to the molten Al to generate gas bubbles. The inclusions adhere to the bubbles and float up together in the molten Al, to separate the inclusions from the molten Al. This method is particularly advantageous for removing gas, such as hydrogen. However, the rising gas bubbles violently wave the molten Al surface and thereby stir the molten Al. This causes another problem that the aluminum oxide film formed on the molten Al surface is likely to be mixed back into the molten Al, resulting in an increased amount of alumina inclusions instead.
According to the molten Al filtering method, the molten Al is allowed to pass through a refractory filter to remove the inclusions therefrom. It is possible to remove relatively large particles of inclusions from the molten Al. However, there is a difficulty of removing small inclusion particles of 100 μm or less, which account for most of the inclusions. In particular, it is difficult to remove fine inclusion particles of about 10 to 25 μm, although such a removal is inevitable to obtain high quality aluminum alloys. In order to remove such fine inclusion particles, the conventional filter needs to have a finer mesh. However, in case that the mesh is too fine, when the molten Al has large amounts of inclusions, there is a practical problem that the filter may be clogged in a short-term operation, resulting in a shortened filter lifetime.
Therefore, there has been a demand for a filtering technology that can simultaneously solve the conflicting conventional problems, i.e., a filtering technology capable of reducing inclusions to a given amount that ensures the required molten Al quality and having a sufficiently long filter life, even when the molten Al is obtained by using scrap of aluminum alloy products as a part or all of the aluminum raw material and thereby the molten aluminum alloy contains a large amount of non-metallic inclusions and, in addition, when the non-metallic inclusions includes fine inclusion particles of about 10 to 25 μm for the most part.
For this purpose, the inventors have proposed a novel filter in Japanese Unexamined Patent Publication Nos. 07-207355A and 09-235629A, instead of the above-mentioned conventional filter that is made only of a refractory aggregate meshed member in which the filtering is performed only by depositing inclusion particles on the filter surface (this type is called “surface filter”). The proposed novel filter includes a refractory aggregate meshed member coated with a compound that is able to be softened or viscous at the temperature of molten aluminum alloy. With this filter, the inclusion particles can be adsorbed by the coating compound (i.e., the coating layer of the aggregate meshed member), to be removed from the molten Al (this type is called “internal filter”).
Specific examples of the coating compound of the internal filter include MnO2, Bi2O3,NaO, B2O3, MgBr2, NaBr, Na2CO3, CrCl2, KCl, NaCl, SrCl2, Na3AlF6, AlK(SO4)2, K2SO4, etc. (Japanese Unexamined Patent Publication No. 09-235629A) and other K or Li alkali metal sulfates, borates, carbonates (K2SO4, Li2SO4, Li2B2O7, Li3CO3) etc.
However, it is necessary to use an adhesive to bond these MnO2 etc. and K or Li alkali metal sulfates etc. (referred to as “conventional coating compounds” in the following) to the surface of the aggregate meshed member, in order to ensure such a strong adhesion therebetween that can resist the impact strength and adhesive strength of the molten Al flow. The reasons are as follows. Any of these conventional coating compounds itself does not have a satisfactorily large adhesion strength with the aggregate meshed member surface. So, when the meshed member is coated only with these compounds (without any adhesive), the compounds may peel off during the filtering, due to the impact strength and adhesive strength of the molten Al flow. This may severely shorten the filter lifetime. Therefore, without using any adhesive, the conventional coating compounds are difficult to apply to a practical molten Al alloy internal filter despite of their excellent adsorbabilities.
As the adhesive to be used together with the conventional coating compounds at a given high temperature, such materials as silica sols is inevitably selected. This is because, it have an excellent thermal resistance when used together with the coating compounds. However, silica sols and other adhesives have an extremely poor inclusion adsorbability, compared with any of the conventional coating compounds (about 15 to 30% of the conventional coating compounds). Therefore, as the adhesive amount increases to ensure the strong adhesion between the coating compound and the aggregate meshed member surface, the coating compound amount decreases by the same amount, resulting in deteriorating the adsorbability of the filter.
Moreover, the filtering ability (efficiency) of the filter to remove inclusions from the molten Al depends mostly on the roughness of the openings formed in the aggregate meshed member and the configuration of the meshed member. Consequently, particularly to remove fine inclusion particles of about 10 to 25 μm, the aggregate meshed member needs to have 2 mesh or more per linear inch (that is, a mesh number of 2 or more).
On the other hand, in order to make the internal filter, an aggregate, for example ceramics, is formed into a meshed shape and baked, to obtain the aggregate meshed member. Then, the aggregate meshed member is impregnated with any one or more of the above-mentioned coating compounds to coat the surface thereof. When the adhesive is used, the meshed member is impregnated sequentially with the adhesive and with the coating compounds. Alternatively, it is impregnated with a mixture of the coating compounds and the adhesive. In any case, the impregnating material, with which the meshed member is impregnated, exhibits considerably high viscosity due to the original physical properties of the coating compounds and the adhesive. The higher the viscosity of the impregnating material is and the finer the mesh of the aggregate meshed member is, the more difficult the coating process becomes. Especially, it is virtually impossible to completely coat not only the meshed member outer surfaces but also the inner surfaces of its openings (pores), even if such surfaces are located in the center of the meshed member, or coat the entire surface of the meshed member constituting a filter.
Consequently, when the above-mentioned conventional coating compounds are applied to the internal filter, the coating efficiency with respect to the entire surface of the meshed member is likely to be low, thereby giving the internal filter a low efficiency of removing inclusions per filter unit volume and filter unit weight. This low removal efficiency may prevent the internal filter from applying to the filtering of molten aluminum alloys.